Add UnitGroupRespawnSystem and upload it on Hive

[tdauth]

Similar to #1006 we need a UnitGroupRespawnSystem which adds all preplaced creeps.

[tdauth]

One big advantage of adding preplaced creep spots as groups is that we get less generated unit variables which will make the trigger editor faster. Besides, the standard item drop system is much simpler and includes all available item types.

[tdauth]

Use as min distance a distance matching creep spots in melee maps.

[tdauth]

Make sure that units and heroes of Bosses players are included into creating respawning unit groups.

[tdauth]

Try to hook generated item Drops from melee creeps. Either Hooker the trigger or the Drop item function from blizzard.j. Once dropped, drop it again in the same way next time. Allow more configutations of item Drops, like itempools, random items etc. Per unit or Last dying from group.

[tdauth]

Add rescuable respawning units to the Demo map.

[tdauth]

Add more API documentation and update it on hive. https://www.hiveworkshop.com/threads/barad%C3%A9s-unit-respawn-system-1-0.341099/

[tdauth]

Finally, the system has been reviewed: https://www.hiveworkshop.com/threads/barad%C3%A9s-unit-respawn-system-1-0.341099/#post-3553198 Adapt to the review and update it.